An organic EL device is a light-emitting diode (LED) in which the light emitting layer is a film made from organic compounds, which emits light in response to an electric current. The light emitting layer containing the organic compound is sandwiched between two electrodes. The organic EL device is applied to flat panel displays due to its high illumination, low weight, ultra-thin profile, self-illumination without back light, low power consumption, wide viewing angle, high contrast, simple fabrication methods and rapid response time.
The first observation of electroluminescence in organic materials was in the early 1950s by Andre Bernanose and his co-workers at the Nancy-University in France. Martin Pope and his co-workers at New York University first observed direct current (DC) electroluminescence on a single pure crystal of anthracene and on anthracene crystals doped with tetracene under vacuum in 1963. The first diode device was created by Ching W. Tang and Steven Van Slyke at Eastman Kodak in 1987. The diode device used a two-layer structure with separate hole transporting and electron transporting layers, resulting in reduction of operating voltage and improvement of the efficiency, thereby leading to the current era of organic EL research and device production.
Typically, organic EL device is composed of organic material layers sandwiched between two electrodes. The organic material layers include the hole transporting layer, the light emitting layer, and the electron transporting layer. The basic mechanism of organic EL involves the injection, transport, and recombination of carriers as well as exciton formation for emitting light. When an external voltage is applied across the organic EL device, electrons and holes are injected from the cathode and the anode, respectively. Electrons will be injected from a cathode into a LUMO (lowest unoccupied molecular orbital) and holes will be injected from an anode into a HOMO (highest occupied molecular orbital). Subsequently, the electrons recombine with holes in the light emitting layer to form excitons and then emit light. When luminescent molecules absorb energy to achieve an excited state, the exciton may either be in a singlet state or a triplet state, depending on how the spins of the electrons and holes have been combined. 75% of the excitons is formed by recombination of electrons and holes to achieve the triplet excited state. Decay from triplet states is spin forbidden, thus, a fluorescence electroluminescent device has only 25% internal quantum efficiency. In contrast to fluorescence electroluminescent device, phosphorescent organic EL device make use of spin-orbit interactions to facilitate intersystem crossing between singlet and triplet states, thus obtaining emission from both singlet and triplet states and the internal quantum efficiency of electroluminescent devices from 25% to 100%. The spin-orbit interactions is achieved by certain heavy atoms, such as iridium, rhodium, platinum, and palladium, and the phosphorescent transition may be observed from an excited MLCT (metal to ligand charge transfer) state of organic metallic complexes.
The phosphorescent organic EL device utilizes both triplet and singlet excitions. Cause of longer lifetime and diffusion length of triplet excitions compared to those of singlet excitions, the phosphorescent organic EL device generally need an additional hole blocking layer (HBL) between the emitting layer (EML) and the electron transporting layer (ETL) or an electron blocking layer (EBL) between the emitting layer (EML) and the hole transporting layer (HTL). The purpose of the use of HBL or EBL is to confine the recombination of injected holes and electrons and the relaxation of created excitons within the EML, hence the device's efficiency can be improved. To meet such roles, the hole blocking materials or the electron blocking materials must have HOMO (highest occupied molecular orbital) and LUMO (lowest unoccupied molecular orbital) energy levels suitable to block hole or electron transport from the EML to the ETL or the HTL.
For full-colored flat panel displays in AMOLED or OLED lighting field, the conventional materials used for the phosphorescent guest in light emitting layer, such as the metallic complexes, are still unsatisfactory in driving voltage, luminous efficiency and half-life time, and still have disadvantages for industrial practice use.
There are some prior-arts disclosing iridium complexes, such as U.S. Pat. No. 8,795,850B2, U.S. Pat. No. 8,778,508B2, U.S. Pat. No. 8,722,205B2, U.S. Pat. No. 8,709,615B2, U.S. Pat. No. 8,779,176B2, but there is no prior art disclosing the iridium complex of the present invention and its use in the organic EL device.